Ramsey FM25


Ramsey's FM25

This is a little transmitting device which covers a small area and is intended to be used in house. You should not couple any amplifying lineair behind this thing, because it's spectrum is not clean.


C1,C2 - 47pf or 20pf ceramic disc (match to 4.00Mhz crystal load imp)
C7,C8,C15,C23,C24, - 0.01uf ceramic disc
C10,C12-C14,C16,C22,C26-C29,C31 - 0.001uf ceramic disc Y5P temp coeff
(note, on the FM100, C16 = 0.1uf ceramic disc for better bass. Must change R10 also.)
C18,C20 - 8.2pf ceramic disc (10pf will work fine too! get COG temp coef version)
C33 - 220pf ceramic disc
C9 - 0.1uf ceramic disc

C3,C4,C6,C17,C19,C21,C25,C34 - 10uf 35V electrolytic (anything above 16V is ok)
C5, C11 - 1000uf 35V electrolytic cap (C11 may be 470uf)

Y1 - 4.000Mhz PARALLEL CUT CRYSTAL (Important! Use a parallel NOT series crystal)
Y2 - 38Khz cystal (available from Digi_key)

INDUCTORS (only one since there is no low pass filter!!)
L1 - 0.085 - 0.100uH adjustable shielded can tunable coil (Mouser part
number 434-1012-3.5CS cost = $0.60)

RESISTORS - all are 1/4W 5% carbon film types
R1,R2,R4,R8,R18,R20,R23-26 - 10K
R2 - 470 ohm
R3 - 1Meg (10Meg can be used too)
R5,R13,R14,R17 - 270 ohm
R7 - 100K
R9,R15 - 68K for US (or 150K), don't know yet for Europe/Asia. Used for Pre-emphasis.
R19 - 2.2K (this resistor sets the non-boosted mode power.)
R10 - 1K (470 ohms is used on the FM100, but must change C16 also)
R12 - 100 ohms
R21 - 4.7K
R22 - 150K

R11,R16 - 1K horizontal mount resistor trimmers
R6 - 100K horizontal mount resistor trimmer

U1 - MC145170P or MC145170P1 PLL chip (available from author or Motorola dist.)
U2 - MC68HC705K1 custom programmed micro-controller (e-mail author for workaround)
U3 - BA1404 Stereo FM transmitter chip (get from DC Electronics for $2.00)

Q1 - 2N3904 NPN general purpose switching transistor (dirt cheap)
Q2,Q3 - 2SC2498 NPN UHF transistors (available from author or MCM electronics)

D1 - Red LED
D2-D5 - 1N4002 diodes (any 1 amp, 50V or better diode will work)
D6-D20 - 1N4148 or 1N914 switching diodes (dirt cheap)
D21 - MV2105 Varactor diode (available from DC Electronics for $0.90)

VR1 - 78L05 +5V 100mA voltage regulator

J1 - DC power jack (DC Electronics, Jameco Electronics)
S1 - DPDT Pushbutton switch (I've seen this blister packed at a local electronics shop)
S2 - Pushbutton switch (available from author, Digi-key, B.G. Micro)
S3 - 3 quantity of a 4 position dip switch. Some FM25 shipped with side actuating 'piano' type levers.
Any 4 position dip switch will work though.
J2-J4 - PC mount RCA jacks (Jameco, Digi-Key, DC Electronics)

In an 'UPDATE' sheet issued by Ramsey, a modification to reduce the 60Hz hum was given.
This modification had the user parallel a 0.01uf ceramic disc capacitor to diodes D2-D5.

The PCB should look like this:

And components should be placed like this:

The FM25 tested was built exactly to the Ramsey assembly instructions. The antenna output jack was used (NO WHIP ANTENNA INSTALLED) and was connected straight to the input of the spectrum analyzer using a short (6") 50ohm coaxial connector using an RCA to BNC adaptor. The input of the spectrum analyzer is 50ohms and can handle 1W of power directly.

The FM25 was installed in its plastic case with the cover removed.

Equipment used : Recently calibrated HP8590E Spectrum Analyzer. This machine is capable of giving direct calibrated readings of amplitude and frequency measurements.

In all these numbers, the amplitude of each fundamental frequency was taken as the reference.

All frequencies are in MHz. All amplitudes are in dB. Only peaks out to about -40dB below fundamental were measured or to the 10th harmonic whichever came first.
non-boosted mode
Freq tested       2nd       3rd       4th       5th
88.1                  15       29.2       42        50
98.0                  16         30        44        55
107.9              17.5        34        49


boosted mode
Freq tested       2nd       3rd        4th       5th       6th       7th       8th       9th      10th
88.1                 10.1      17.6       18        26       24        32       28.8    34.6     36.4
98.0                  12         16         20        22       28        28        34        36        38
107.9                15        15.8       28        23       30       30.7      36        39        41


So, as you can see, the Ramsey FM25 in boosted mode spews a LOT of harmonics! The non-boosted mode is much cleaner, but from the numbers above, if you plan on hooking up a decent antenna, or plan on driving an amplifier stage, adding a low pass filter is a must!

The numbers above also show that the FM25 puts out more power at lower frequencies. In fact here are the measured amplitude number for the fundamentals.

non-boosted mode (R19 = 2.2K)

Frequency       amplitude measured            mW equivalent
88.1                      +3.0 dBm                          2.0 mW
98.0                        2.4 dBm                         0.58 mW
107.9                      4.0 dBm                         0.40 mW

I remeasured the FM25 after replacing R19 with a 750 ohm resistor (stock value is 2.2K). I did this in an attempt to boost the output of the FM25 without overdriving the final RF output transistor. The resulting spectral output is still pretty bad. Sidebands appear and the fundamental peak starts to get wide. Harmonics were boosted also.
Here are the measured amplitudes of the fundamentals for the FM25 in boosted mode, and R19 = 750 ohms non-boosted mode:

Frequency           Jumpered         mW Equiv            R19 = 750ohms               mW Equiv
88.1                    15.6 dBm         36.3 mW                 8.0 dBm                         6.3 mW
97.9                    15.5 dBm          35.5 mW                6.8 dBm                         4.8 mW
107.9                  15.2 dBm           33.1 mW               5.6 dBm                         3.6 mW

Here are the amplitudes for the harmonics relative to the fundamental amplitude for the non-jumpered, R19 = 750 ohms configuration.
non-boosted mode, R19 = 750 ohms

Freq tested             2nd        3rd       4th       5th
97.9                       12.6      28.2     36.0      44.0

All measurements are in dB. The amplitude measurements for the jumpered configuration don't change much with frequency, mostly due to the output being overdriven. In the R19 = 750ohms configuration, you can see the FM25 puts out more power at lower frequencies (like the stock configuration).


dschmidt (dschmidt@rain.org)






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